This section is intended to provide a background or context to the invention recited in the claims. The description herein may include concepts that could be pursued, but are not necessarily ones that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, what is described in this section is not prior art to the description and claims in this application and is not admitted to be prior art by inclusion in this section.
Lenses are an integral part of a large number of optical systems, including microscopes, cameras, telescopes, spectrometers, and so on. Conventionally, lenses designed for the visible wavelengths are made of blocks of glass whose surfaces are polished to be curved in a specific manner, making them incompatible with large-volume semiconductor fabrication processes. For example, cameras and similar devices include such glass lenses. There exists an intrinsic limit to how sleek the glass lenses can be, and major drawbacks associated with glass lenses in small-scale items such as cell phone cameras include the size and bulkiness of such lenses. Lighter and thinner lenses are desirable that would be comparable in efficiency to conventional glass lenses.
The curvature of the glass piece in an optical lens, the curvature defined as the varying thickness across the cross-section of the glass piece in the lens, determines how the light passing through the lens will bend and eventually come to a focal point. In the past decade, as nanofabrication technologies developed, researchers have been exploring ways to bend light by using specifically designed nanostructures arranged on a flat surface.
Conventional nanostructured planar devices are based on metal-dielectric nanostructures. A dielectric material is a substance that is a poor conductor of electricity but an efficient supporter of electrostatic fields. Most dielectric materials are solid. Examples include porcelain (ceramic), mica, glass, plastics, and the oxides of various metals.
Problematically, metals have inherently high dielectric losses for the visible wavelength spectrum. Dielectric loss refers to a dielectric material's inherent dissipation of electromagnetic energy (e.g. heat). It can be parameterized in terms of either the loss angle delta (δ) or the corresponding loss tangent of delta (tan δ). Dielectric loss occurs through conduction, slow polarization currents, and other dissipative phenomena. Because of dielectric loss, metals have poor transmission efficiency across the visible wavelength range.
Conventionally, early-generation nanostructured planar optical devices are based on metal-dielectric nanostructures. However, because metals have inherently high losses for the visible wavelengths, metal-dielectric nanostructures offer poor transmission efficiency across the visible wavelength range. An alternative is an all-dielectric lens. However, although all-dielectric lenses exhibit high efficiency, they are based on tall dielectric pillars with high aspect ratios, and are sensitive to the polarization of incoming light. This makes such all-dielectric flat devices unsuitable for large-scale manufacturing, and certainly not viable for generic (unpolarized) optical applications.
Light is electromagnetic radiation within a certain portion of the electromagnetic spectrum. The wavelength of visible light ranges from roughly 400 nm to roughly 700 nm. Huygens resonators are sub-wavelength dielectric structures for modulating the amplitude and phase of a signal carried by a light beam. Sub-wavelength means that Huygens resonators have dimensions less than the wavelength of the light employed. Huygens resonators form Huygens metasurfaces. Metasurfaces are slabs of sub-wavelength thickness containing sub-wavelength in-plane features (meta-atoms) that are used to realize a desirable functionality by local modification of the interaction between the slab and incident electromagnetic fields. In recent years, such structures have attracted significant attention due to their potential to provide excellent control on properties of transmitted or reflected fields, such as directivity, polarization and orbital angular momentum, with low-profile conformal devices.
Huygens metasurfaces utilize collocated orthogonal electric and magnetic polarizable elements. Problematically, because Huygens resonators are sub-wavelength dielectric particles, conventional Huygens metasurfaces are not generally functional throughout the visible spectrum, including blue, green, and red wavelengths. In the context of the present disclosure, “functional” is defined as capable of bending light (changing the phase of the propagating light) and enabling it to come to a focal point so that an optical device comprising the Huygens metasurface can act as a lens with high transmission efficiency.
Recently, researchers started investigating Huygens surfaces for visible wavelengths. Silicon was their obvious material of choice, but silicon failed to deliver sufficiently low dielectric loss at the points in the spectrum other than near-infrared wavelengths, as shown, for example, in FIG. 1. The dielectric loss of silicon is enough to not allow 100% intensity. Thus, functional Huygens surfaces that include other dielectric materials are desired to bypass this limitation.